The present invention relates to earth boring tools or augers, and more particularly to that portion of the auger which encounters the earth first, commonly referred to as a pilot bit, for breaking the way for larger portions of the auger.
Pilot bits generally are provided at the drilling end of an auger to guide the auger and keep it boring in the correct direction. Heretofore, flighted pilot bits have included two flutes or blades which are arranged in a double helix flighting configuration. Each blade extends radially from a core member with its outer edge forming a spiral cutting surface. With this configuration, two paths are defined between the blades for spoil to flow as the pilot bit is rotated to penetrate into the earth. Arrangements for releasably securing these bits have been developed to facilitate replacement of the bit when its cutting edges are worn out. One such arrangement is disclosed in U.S. Pat. No. 2,870,995 in which a drive lug is welded onto the shaft of the auger for receipt in a cavity formed in the pilot bit. The lug is bolted to the core member of the pilot bit and extends into the paths or channels formed between the pilot bit blades.
Although the two fluted pilot bit configuration provides large paths for spoil to flow, it has been found that this configuration results in undesirable drift. It is believed, that with only two cutting edges, if an obstruction or change in the earth formation being excavated is encountered, the bit can shift laterally since it is only supported by two diametrically opposed cutting edges. In addition, with the drive lug arrangement in which the bolt and nut extend through the pilot bit core member and beyond its surface, the bolt and nut act as obstructions to spoil flow along the outer surface of the core member. This reduces drilling rates since the bit only can drill as fast as the spoil is removed from the hole. The constant flow of the spoil over the nut also wears out the nut and bolt threads so that it is difficult to remove the nut and replace the pilot bit.